1. Field of the Invention
The present invention relates to a single endsealed metal halide lamp, more particularly, to a small metal halide lamp which can be lit with a high load.
2. Description of the Related Art
A high intensity discharge lamp (HID), which was widely used for outdoor illumination or illumination within a factory, has come to be used/recently for the indoor illumination in the architecture where there are low ceilings, such as stores. Among the HID, a metal halide lamp, which is miniaturized, is used in many cases in, for example, stores because the metal halide lamp exhibits a high efficacy and high color rendering properties.
A conventional metal halide lamp is sealed at both ends of an arc tube bulb. Since the bulb is sealed by pinch sealing at both ends, the manufacturing process of the lamp is complex. Also, since the area of the sealed portion is large relative to the entire area of the arc tube bulb, the lamp is rendered bulky as a whole. Further, heat is transmitted to the sealed portion via the wall defining the discharge space, and the heat is released from the sealed portion to the outside. It follows that the conventional metal halide lamp gives rise to a large heat loss.
In order to miniaturize the lamp, it is advantageous to form a sealed portion at one end of a bulb and the discharge space is surrounded by the other end of the bulb, as disclosed in Japanese Patent Disclosure 60-232662. A lamp of this type is low in heat loss because the arc tube bulb is sealed at only one end, leading to an improved light emitting efficiency. Also, the sealing process can be simplified because only a single end of the bulb is sealed. In addition, the single end-sealed type makes it possible to miniaturize the lamp as a whole.
The single end-sealed arc tube bulb comprises a pair of metal foils, e.g., molybdenum foils, buried in one end of the bulb and rod electrodes each having one end connected to the foil. The other end of each rod electrode extends into the discharge space and an electrode coil is mounted to the other end of the rod electrode.
The conventional metal halide lamp of single endsealed type is lit under a high lamp load, i.e., the value of WL/S falls within the range of between 20 and 70, where WL denotes the lamp input power (W) and S represents the inner surface area (cm.sup.2) of the discharge space. In the lamp lit under such a high load, however, the initial blackening of the light emitting bulb is promoted, leading to reduction in the lumen maintenance factor and to a shortened life of the lamp.
In the conventional lamp, each of the electrode, electrode rod and electrode coil is formed of a tungsten wire or a thoriated tungsten (ThO.sub.2 -W) wire. In the step of mounting a pair of electrodes in the sealed portion at one end of the arc tube bulb, the electrodes connected to the pair of metal foils are inserted into the bulb through one open end of the bulb. Under this condition, the open end of the bulb is softened under heat, followed by pinching under pressure the softened end portion so as to achieve the sealing. It should be noted that the electrode rods are positioned relatively close to the wall of the arc tube bulb in the case of inserting two electrodes into the bulb, compared with the case of inserting a single electrode rod. Thus, in the step of softening the open end portion of the bulb under heat, the two electrode rods are heated to a high temperature by the bulb wall, leading to oxidation of the electrode rods. When oxidized, the material of the electrode rod tends to be scattered. The scattered electrode material is attached to the inner surface of the bulb and, thus, the bulb wall is blackened. Since the surface area of the arc tube bulb is small in a lamp of this type, the blackening proceeds promptly in a short time, even if the electrode material is scattered only in a small amount, resulting in a marked reduction in the lumen maintenance factor.
The conventional electrode formed of tungsten or thoriated tungsten has a relatively high heat conductivity. Thus, heat is conducted to reach the proximal end portion of the electrode rod during the lighting of the lamp, leading to heating to a high temperature. In addition, since the bulb is small and sealed at one end alone, the distance is small between the proximal end portions of the pair of the electrode rods. In conjunction with the heating to a high temperature noted above, the construction noted above causes the arc spot to be moved to a space between the proximal end portions of the electrode rods, with the result that discharge is likely to take place easily between the electrode rods. Such a discharge causes the material of the electrode rod to be scattered, leading to breakage of the electrode rod and to a short life of the lamp.
On the other hand, an arc spot is formed during the lighting between a pair of coils disposed to face each other within the discharge space so as to generate an arc discharge between these coils. The arc spot tends to move easily during the lighting of the lamp. The movement of the arc spot causes a change in the distance between the arc and the inner surface of the arc tube bulb. Naturally, changes also take place in the position and temperature of the coolest region of the bulb wall rearward of the electrode coil. As a result, the lamp voltage is changed and the light emitting characteristics are rendered nonuniform. The movement of the arc spot brings about additional problems. For example, the arc is positioned close to inner surface of the arc tube bulb so as to locally heat the light emitting bulb to a high temperature and, thus, to cause a thermal expansion of the bulb.